1. Field of the Invention
The present invention relates to a method for producing a composite component, to a composite component which is produced by applying said method, and to a turbomachine, in particular a gas turbine, comprising such a composite component.
2. Brief Description of the Related Art
Electro discharge machining (EDM) and electro chemical machining (ECM) are well-known methods of machining. EDM and ECM are generally used to machine a workpiece in which high precision is required in machining high-hardness materials, at the same time avoiding local thermal load during the machining step, such as, for example, in machining a component for a turbine. The methods are in particular used to produce cooling holes, such as the holes through which film cooling air flows during operation.
In EDM at least one electrode is held in close proximity to the workpiece and electrical sparks are generated between the workpiece and the electrode due to a difference in electrical potential. The sparks cause the material of the workpiece to erode. In ECM a workpiece acts as an electrode which is coupled electrically to a further electrode by means of an electrolyte. Especially with ECM freeform geometries can be machined.
It is thus clear that the surface of the workpiece must be electrically conductive in order to use EDM or ECM. Therefore, difficulties arise when it is necessary to machine a workpiece formed from a composite. Such composites often contain electrically non-conductive materials, such as ceramics. It is particularly common for one or more electrically non-conductive materials to be provided as a coating on or around a metal substrate in order to protect the metal substrate from, for example, heat, corrosion, chemical corrosion, and/or wear during use. This is highly relevant to industries such as the aerospace and power generation industries, where turbine blades are required to operate at very high speeds and temperatures.
Several methods have been proposed in the art to facilitate the use of electro machining methods with composites including electrically non-conductive materials, such as coatings.
EP 0 366 466 discloses a method of EDM in which a composite workpiece is first machined using ultrasonic drilling, in order to remove an amount of electrically non-conductive material, and so to provide access to an underlying electrically conductive material. EDM can then be carried out on the electrically conductive material in the usual manner. A disadvantage of this technique is that the ultrasonic drilling can only be carried out perpendicular to the surface to be machined, which limits its applicability to only certain structural configurations for the workpiece.
U.S. Pat. No. 5,177,037 describes types of composite ceramics which have been rendered electrically conductive by the inclusion of metal and/or carbon. It is also known to dope electrically non-conductive materials with a suitable electrolytic fluid, in order to render the materials electrically conductive. EDM can then be applied to the electrically conductive composite. However, doping of the material in this way can affect the integrity of the coating, which is obviously undesirable. Furthermore, substantial washing of the material is required to remove traces of the dopant. This complicates the manufacturing process considerably. The concentration and distribution of the dopant in the material is relatively difficult to control. Thus, it is possible that the inclusion of the dopant will not achieve the necessary electrical conductivity
From DD 274 999 and U.S. Pat. No. 4,818,834, among others, methods are known where laser drilling methods are used to machine a non-conductive ceramic coating to expose the conductive substrate, which is then available for electro machining, such as ECM or EDM. However, applying conventional laser drilling methods the achievable geometry is limited to basically circular cross sections, including an elliptical opening on the component surface, subject to the angle of incidence of the laser beam on the surface. Furthermore, with the known technique, the diameter of the hole to be produced is essentially restricted to the laser beam diameter in the working range. Moreover, due to the high thermal energy applied, and due to the fact that the working range of the drilling laser, may extend into the metallic substrate, also the metallic substrate is at least partly, subject to some extent to laser machining, and is at least subject to local thermal loading. As a consequence, the structure of a metallic substrate is subject to highly undesirable structural transformations. The adherence between a laser machined coating and a metallic substrate is locally severely weakened, giving rise to potential subsequent damages: In the end, current laser drilling methods are after all thermal machining methods, and this is exactly what shall initially be avoided in applying electro machining methods. The precision of the geometry is limited, and currently, for example in producing cooling air holes, tolerances in the cooling air mass flow through a hole due to the manufacturing process of the cooling holes, are in an order of magnitude of ±10% of a mean mass flow, in spite of the high precision achievable by electro machining.
Summarizing, although the combined manufacturing process described above, is widely used and was found to work all in all satisfactory, latest developments, in particular in gas turbine technology, have lead to increasing demands for precise cooling air control and best mechanical component integrity, and have thus triggered a strong desire for improvement to avoid or at least minimize the drawbacks related to the present art cited above.